As the number of devices in a very large scale integrated (VLSI) circuit chip increases, the individual transistor devices which form its component elementary logic circuits, occupy areas of the chip on the order of a few square microns or less. The quantity of charge which is transferred between field effect transistor devices of that size while carrying out normal switching operations is such that the circuits are very susceptible to electrostatic perturbations.
One ubiquitous source of such perturbations is cosmic energy. Cosmic rays are a highly penetrating form of radiation, which apparently come in all directions from outer space to reach the earth. The primary cosmic rays entering the earth's atmosphere are almost entirely composed of positively charged atomic nuclei which collide with air nuclei high in the atmosphere, thereby forming showers of positively and negatively charged nuclear fragments called secondary cosmic rays. The secondary cosmic rays penetrate all matter at the earth's surface and, as they pass through a material object, they undergo collisions with electrons and nuclei of which the material object is composed, leaving a track of electrostatic charge along the way. The linear charge density along such a track can be typically three-tenths (0.3) Pico-Coulombs per micron, which is on the same scale as the quantity of charge involved in the switching operation of a single field effect transistor on a VLSI chip.
This phenomenon becomes a significant problem in logic circuits composed of such devices since logic circuits must sense logic states and provide outputs based upon such sensed logic states. When semiconductor devices such as the transistors which make up logic circuitry are struck by alpha particles for example, they may experience so called soft errors which may affect a high logic signal and cause it to go to a low logic level, or vice versa, thereby compromising the integrity of the entire logic circuit.
In the past, similar problems have been recognized for storage devices but the solutions to such problems have heretofore been in the form of altering the materials and structures of the semiconductor storage devices and have not been applicable to logic circuitry. Cosmic and alpha particles, also pose a problem to the integrity of dynamic memory storage cells. When an alpha particle strikes a chip at the cell location and causes the charge stored in the cell to be modified such that the stored logical state is changed. Considerable efforts have been made to avoid this type of problem but memory correction schemes have been the primary solution.
The same electrical phenomena occur in CMOS dynamic logic circuits where a signal is stored on the parasitic capacitance of the circuit precharge node. The solutions for memory error correction can not be applied in the case of CMOS dynamic logic circuits and other solutions must be found.
Thus there is a need for a method and apparatus which is effective to increase the reliability of logic circuit devices and reduce the potential for error in such devices which may result from one-time electrostatic events such as an alpha particle and other cosmic energy particle collisions with such devices.